Exhaust Gas Purifier

ABSTRACT

It is intend to provide at low cost an exhaust gas purifier capable of in an internal combustion engine or other combustion instrument mainly driven under excess air conditions, efficiently removing nitrogen oxides from exhaust gas. A nitrogen oxide adsorbent ( 4 ) is disposed in an exhaust passage, which the nitrogen oxide adsorbent ( 4 ) is composed of a lithium composite oxide of the general formula LiAxOy or LiAxPO 4 , containing as constituent elements lithium (Li) and at least one element (A) selected from the group consisting of manganese (Mn), nickel (Ni), cobalt (Co), vanadium (V), chromium (Cr), iron (Fe), titanium (Ti), scandium (Sc), and yttrium (Y). For example, the nitrogen oxide adsorbent ( 4 ) is composed of a lithium composite oxide, such as lithium manganate (LiMnO 3 ), the lithium titanate (Li 2 TiO 3 ) or lithium manganate phosphate (LiMnPO 4 ).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of application Ser. No.11/887,118, filed Sep. 25, 2007, which is a National Phase ofInternational Application No. PCT/JP2006/304996, filed Mar. 13, 2006,which claims priority under 35 U.S.C. §119 to Japanese PatentApplication No. JP 2005-093798, filed Mar. 29, 2005, and Japanese PatentApplication No. JP 2005-093802, filed Mar. 29, 2005, the entiredisclosures of which are herein expressly incorporated by reference.

TECHNICAL FIELD

The present invention relates to an apparatus for purifying exhaust gasof an internal combustion engine such as a diesel engine, a gas engine,a gasoline engine, and a gas turbine engine or a combustion device suchas an incinerator and a boiler. More particularly, the present inventionrelates to an exhaust gas purifier suitably removes nitrogen oxide whilebeing installed within an exhaust gas passage of the internal combustionengine or the like which is normally operated in an excessive air state.

BACKGROUND ART

A target substance subject to exhaust gas purification is a particulatematter such as nitrogen oxide, carbon monoxide, unburned hydrocarbon,soot and the like. Various apparatuses for purifying these substanceshave been conventionally developed.

As an apparatus for reducing the nitrogen oxide (NOx), a denitrationapparatus or the like has come into practical use, in which a reductioncatalyst using an ammonia or an urea as a reducing agent is installed inan exhaust passage, thereby selectively reducing the nitrogen oxide.Further, in a comparatively compact gas engine or an automotive gasolineengine, there has been developed a three-way catalyst which cansimultaneously decompose three elements comprising the nitrogen oxide,the carbon monoxide (CO) and the unburned hydrocarbon (HC), and thethree-way catalyst contributes to an effective purification of theexhaust gas.

However, it has been known that the three-way catalyst effectivelyachieves a purifying operation in the case that the three-way catalystis operated at a theoretical air fuel ratio or within a range closethereto, but is not effectively operated under the other conditions,particularly in exhaust gas in which air (oxygen) is excessive.

In order to cope with this, in the gas or gasoline engine operated inthe excessive air state, a nitrogen oxide occluding catalyst system hascome into practical use, which temporarily occludes the nitrogen oxidein an occluding agent at a time of being operated under the excessiveair (oxygen) condition, and next discharges and reduces the occludednitrogen oxide by being operated under the excessive fuel condition.

Conventionally, various kinds of combinations of noble metal andalkaline metal oxide or alkaline-earth metal oxide have been developedas the nitrogen oxide occluding type catalyst (for example, see PatentDocument 1). In such catalysts, during excessive air combustion (leanburn) of a normal operation, an NO in the exhaust gas is oxidized toobtain an NO₂ on the noble metal catalyst, the NO₂ is caused to reactwith a basic oxide of the alkaline metal or alkaline-earth metal, andthe NOx is occluded in the form of a nitrate. During excessive fuelcombustion (rich operation) of a regenerating operation, the NO₂desorbed from the oxide is reduced to obtain an N₂ by reducing substancesuch as hydrocarbon and the Co on the noble metal catalyst, and thedetoxified nitrogen is discharged.

FIG. 15 is a graph showing a ratio of substances discharged from thenitrogen oxide occluding catalyst by the regenerating operation in thecase of the use of the nitrogen oxide occluding type catalyst formed bythe combination of the noble metal and the alkaline metal oxide oralkaline-earth metal oxide. About 75% of the substance is discharged inthe form of the N₂ and remaining 25% is discharged in the form of theNOx such as the NO, the N₂O, and the NO₂.

-   Patent Document 1: Japanese Patent Publication laid-Open No.    2001-000863

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

A denitration apparatus which selectively reduces the nitrogen oxideusing ammonia or urea, and the like is applied to a relatively largeindustrial internal combustion engine or a combustion instrument.However, the apparatus itself is large and extremely expensive, and amaintenance cost of the reducing agent such as ammonia and urea is alsoincreased. Further, there is a high possibility that the unconsumedammonia is discharged into atmosphere.

As described above, the three-way catalyst cannot exert a catalystfunction in the internal combustion engine or the combustion instrumentoperated under the excessive air condition.

In the nitrogen oxide occluding type catalyst which is commercialized inthe small gas engine and automobile gasoline engine, the purifierbecomes expensive because the noble metal is always contained as thecatalyst component, an operation range (such as temperature and SVvalue) where the reduction can completely be performed is restricted toa narrow range, and complicated lean and rich control (control of airand fuel supply amounts) is required on an engine side.

Object of the Invention

An object of the invention is to provide an exhaust gas purifier whichcan efficiently remove and detoxify the nitrogen oxide in the exhaustgas and discharge the nitrogen to the atmosphere at low cost in theinternal combustion engine or combustion instrument which is operatedunder the excessive air condition. Another object of the invention is tosuppress deterioration of the nitrogen oxide adsorbent, caused bypoisoning of sulfur, to sufficiently exert performance even in the fuelcontaining a large amount of sulfur component.

Problems to be Solved by the Invention

In order to solve the problem, a first aspect according to the presentinvention provides an exhaust gas purifier which is installed in anexhaust passage of an internal combustion engine or a combustioninstrument, the exhaust gas purifier characterized in that a nitrogenoxide adsorbent is disposed in the exhaust passage, and the nitrogenoxide adsorbent is made of lithium composite oxide expressed by ageneral formula LiAxOy or LiAxPO₄, where A is at least one kind ofelement selected from an element group of manganese (Mn), nickel (Ni),cobalt (Co), vanadium (V), chromium (Cr), iron (Fe), titanium (Ti),scandium (Sc), and yttrium (Y), and the at least one kind of element Aand lithium (Li) are elements constituting the general formula LiAxOy orLiAxPO₄. For example, the nitrogen oxide adsorbent is made of thelithium composite oxide such as lithium manganate (LiMn₂O₄), lithiumtitanate (Li₂TiO₃), and lithium manganate phosphate (LiMnPO₄),

Currently the composite oxide of the lithium (Li) of the alkaline metaland transition metal such as manganese (Mn) is produced and distributedin large quantity in the market as a positive electrode material forlithium-ion battery, so that the composite oxide can easily be obtainedat low cost. Therefore, cost reduction of the apparatus cost can beachieved. Additionally, the composite oxide reacts easily with latticeoxygen because the composite oxide contains the transition metal such asmanganese (Mn), the composite oxide rapidly oxidizes the NO to obtainthe NO₂, and the saturated NOx adsorbed amount is increased, so that anNOx adsorbing power is improved.

In the exhaust gas purifier according to the first aspect of theinvention, a second aspect according to the invention is characterizedin that noble metal is added to the nitrogen oxide adsorbent.

When the noble metal is added, the apparatus cost is increased comparedwith the case in which the noble metal is not contained. However, thepositive electrode material for lithium-ion battery which is distributedat low cost in large quantity in the market is utilized, which allowsthe NOx adsorbing power to be improved while the cost increase issuppressed for the overall of the nitrogen oxide adsorbent.

In the exhaust gas purifier according to the second aspect of theinvention, a third aspect according to the invention is characterized inthat the noble metal is platinum (Pt) and the lithium composite oxide islithium titanate (Li₂TiO₃).

The use of the lithium titanate (Pt—Li₂TiO₃) to which platinum is addedas the noble metal improves a SOx-resistant property to prevent thepoisoning of the nitrogen oxide adsorbent, which lifetime to belengthened.

In the exhaust gas purifier as in any one of the first to third aspectsof the invention, a fourth aspect according to the invention ischaracterized in that the nitrogen oxide adsorbent is supported by asupport made of aluminum oxide (Al₂O₃) and/or anatase titanium oxide(TiO₂).

When the nitrogen oxide adsorbent is supported by the porous supporthaving a large specific surface area, the NOx adsorbing power is furtherimproved. Note that the anatase titanium oxide (TiO₂) having the largespecific surface area is stable at low temperatures against rutiletitanium oxide which is stable at high temperatures.

In the exhaust gas purifier as in any one of the first to fourth aspectsof the invention, a fifth aspect according to the invention ischaracterized in that an additive amount as lithium oxide (Li₂O) rangesfrom 10 to 20 weight % in the nitrogen oxide adsorbent.

When the lithium oxide (Li₂O) is added with the weight proportiondescribed above, the NOx adsorbing power is further improved.

In the exhaust gas purifier as in any one of the first to fifth aspectsof the invention, a sixth aspect according to the invention ischaracterized in that the nitrogen oxide adsorbent is burned in a rangeof 400° C. to 500° C.

By burning the nitrogen oxide adsorbent in the above temperature range,the large specific surface area can be ensured while the saturated NOxadsorbed amount is increased, and thus the NOx adsorbing power canfurther be improved. Most preferably the nitrogen oxide adsorbent isburned at 450° C.

In the exhaust gas purifier as in any one of the second to sixth aspectsof the invention, a seventh aspect according to the invention ischaracterized in that adsorbed substance desorbing means is disposed onan exhaust gas upstream side of the nitrogen oxide adsorbent and ancombustion apparatus is disposed on an exhaust gas downstream side ofthe nitrogen oxide adsorbent.

According to the above-mentioned configuration, during the normaloperation of the internal combustion engine, particularly during thenormal operation under the excessive air condition, the nitrogen oxidesuch as the NO and the NO₂ is easily generated, and the generatednitrogen oxide such as the NO and the NO₂ is temporarily adsorbed to thenitrogen oxide adsorbent. When the NOx adsorbed amount reaches apredetermined amount, the nitrogen oxide adsorbing means and thecombustion apparatus are operated to perform the regenerating operation.During the regenerating operation, using the adsorbed substancedesorbing means, the nitrogen oxide adsorbent is heated or the gas isconverted into the reducing atmosphere, and the nitrogen oxide isdesorbed. In the case where the noble metal is not contained in thenitrogen oxide adsorbent, most nitrogen is desorbed in the form of thenitrogen oxide such as the NO and the NO₂, and the desorbed NOx isreduced into the N₂ in an over-rich combustion region on the downstreamside of the combustion apparatus and detoxified and discharged to theatmosphere.

In the case where the CO and hydrocarbon are generated in the over-richcombustion region of the adsorbed substance desorbing means orcombustion apparatus during the regenerating operation, the CO andhydrocarbon are oxidized to obtain a CO₂ and an H₂O in a lean fuelcombustion region of the combustion apparatus and detoxified anddischarged to the atmosphere. Because the combustion apparatus has lowcombustion temperatures in the lean fuel combustion region, the N₂desorbed by the adsorbed substance desorbing means is not oxidized againnor returned to the nitrogen oxide.

Thus, when the combustion apparatus is disposed on the downstream sideof the nitrogen oxide adsorbent, similar to the case where the use ofthe catalyst containing the noble metal, the nitrogen oxide in theexhaust gas can economically be detoxified and discharged to theatmosphere even by the inexpensive nitrogen oxide adsorbent which doesnot contain the noble metal. Additionally, the normal operation and theregenerating operation can be performed without performing thecomplicated lean and rich control on the engine side.

In the exhaust gas purifier as in any one of the second to seventhaspects of the invention, an eighth aspect according to the presentinvention is characterized in that the combustion apparatus is a fuellean-burn type.

According to the above-mentioned configuration, in the case where thenitrogen oxide adsorbent contains the noble metal, because mostsubstances desorbed from the nitrogen oxide adsorbent are reduced intothe N₂ by the catalytic action of the noble metal, the combustionapparatus which performs the lean fuel combustion to detoxify only theCO and the hydrocarbon can be used as the combustion apparatus disposedon the downstream side. Therefore, fuel consumption can be saved.

In the exhaust gas purifier as in any one of the third to seventhaspects of the invention, a ninth aspect according to the invention ischaracterized in that the adsorbed substance desorbing means is set to atemperature near the burning temperature or to the burning temperatureor less. For example, when the burning temperature is 450° C., thecombustion is controlled so as to be performed at a temperature near450° C. or a temperature at 450° C. or less.

As with the above-mentioned configuration, when the temperature issuppressed to the temperature near the burning temperature or theburning temperature or less, sintering and destruction by fire of thelithium (Li) are prevented, which allows performance deterioration to besuppressed to lengthen a lifetime of the nitrogen oxide adsorbent.

In the exhaust gas purifier according to the first aspect of theinvention, a tenth aspect according to the invention is characterized inthat a sulfur oxide adsorbent is disposed on an exhaust gas upstreamside of the nitrogen oxide adsorbent.

According to the above-mentioned configuration, during the normaloperation of the internal combustion engine, because the sulfur oxide inthe exhaust gas is adsorbed to the sulfur oxide adsorbent beforereaching the nitrogen oxide adsorbent, the nitrogen oxide adsorbent isnot poisoned by the sulfur, and the decrease in nitrogen oxide adsorbedamount caused by the sulfur poisoning can be prevented. In addition,durability is also improved. Particularly, the lithium composite oxide(for example, LiMn₂O₄) made of a transition metal element except for thetitanium (Ti) has a lower SOx absorbing property compared with thelithium composite oxide containing the titanium (Ti), the poisoning ofthe nitrogen oxide adsorbent can be prevented by disposing the sulfuroxide adsorbent as described above.

In the exhaust gas purifier according to the tenth aspect of theinvention, an eleventh aspect according to the invention ischaracterized in that the sulfur oxide adsorbent contains copper oxideand zirconium oxide.

According to the above-mentioned configuration, because the copper oxideand zirconium oxide have the good sulfur oxide (SOx) absorbing power,the sulfur oxide adsorbed amount can be increased.

In the exhaust gas purifier according to the eleventh aspect of theinvention, a twelfth aspect according to the invention is characterizedin that a metal ratio of copper and zirconium is 1:1 in the sulfur oxideadsorbent.

According to the above-mentioned configuration, the sulfur oxideadsorbed amount can be increased.

In the exhaust gas purifier according to the eleventh or twelfth aspectof the invention, a thirteenth aspect according to the invention ischaracterized in that adsorbed substance desorbing means is disposed onan exhaust gas upstream side of the sulfur oxide adsorbent and ancombustion apparatus is disposed on an exhaust gas downstream side ofthe nitrogen oxide adsorbent.

The copper oxide and zirconium oxide can reversibly adsorb and desorbthe sulfur oxide. Therefore, in the exhaust gas purifier in which thenitrogen oxide desorbing means provided on the upstream side of thenitrogen oxide adsorbent while the combustion apparatus is provided onthe downstream side, by switching between the normal operation and theregenerating operation, the nitrogen oxide is adsorbed to the nitrogenoxide adsorbent and the sulfur oxide is simultaneously adsorbed to thesulfur oxide adsorbent during the excessive air combustion of the normaloperation, and then the nitrogen oxide and sulfur oxide adsorbed to theadsorbents are desorbed during the over-rich combustion of theregenerating operation, and the nitrogen oxide can be detoxified on thedownstream side of the combustion apparatus and discharged to theatmosphere.

A fourteenth aspect according to the invention provides an exhaust gaspurifier which is installed in an exhaust passage of an internalcombustion engine or a combustion instrument, the exhaust gas purifierincluding a nitrogen oxide adsorbent which temporarily adsorbs nitrogenoxide even in an excessive air atmosphere and desorbs the adsorbednitrogen oxide by heating or reducing atmosphere; adsorbed substancedesorbing means which is disposed on an exhaust gas upstream side of thenitrogen oxide adsorbent, the adsorbed substance desorbing means heatingthe exhaust gas or air or converting the exhaust gas or air into areducing atmosphere; and a combustion apparatus which is disposed on anexhaust gas downstream side of the nitrogen oxide adsorbent, wherein thenitrogen oxide adsorbent is made of metal oxide which does not containnoble metal.

In the exhaust gas purifier according to the fourteenth aspect of theinvention, during the normal operation of the engine, particularlyduring the normal operation under the excessive air condition, thenitrogen oxide such as the NO and the NO₂ is easily generated, and thegenerated nitrogen oxide such as the NO and the NO₂ is temporarilyadsorbed to the nitrogen oxide adsorbent. When the NOx adsorbed amountreaches a predetermined amount, the nitrogen oxide adsorbing means andthe combustion apparatus are operated to perform the regeneratingoperation. During the regenerating operation, using the adsorbedsubstance desorbing means, the nitrogen oxide adsorbent is heated or thegas is converted into the reducing atmosphere, and the nitrogen oxide isdesorbed. Because the noble metal is not contained in the nitrogen oxideadsorbent, most nitrogen is desorbed in the form of the nitrogen oxidesuch as the NO and the NO₂ while a little amount of nitrogen is desorbedin the form of the N₂, and the desorbed the NOx is reduced into the N₂in the over-rich combustion region of the combustion apparatus on thedownstream side of the nitrogen oxide adsorbent and detoxified anddischarged to the atmosphere.

On the other hand, the CO and the hydrocarbon generated in the over-richcombustion region of the combustion apparatus is oxidized into the CO₂in the excessive air combustion region of the combustion apparatus anddischarged to the atmosphere. Because of the low temperature in theexcessive air combustion region, the N₂ desorbed by the adsorbedsubstance desorbing means is not oxidized again nor returned to thenitrogen oxide.

Thus, when the combustion apparatus is disposed on the downstream sideof the nitrogen oxide adsorbent, similar to the case where the use ofthe catalyst containing the noble metal, the nitrogen oxide in theexhaust gas can economically be detoxified and discharged to theatmosphere even by the inexpensive nitrogen oxide adsorbent which doesnot contain the noble metal. Additionally, the normal operation and theregenerating operation can be performed without performing thecomplicated lean and rich control on the engine side.

In the exhaust gas purifier according to the fourteenth aspect of theinvention, a fifteenth aspect according to the invention ischaracterized in that the nitrogen oxide adsorbent is made of transitionmetal oxide.

As with the above-mentioned configuration, the use of the transitionmetal oxide can easily react with the lattice oxygen to rapidly oxidizethe NO into the NO₂. Therefore, the saturated NOx adsorbed amount andthe NOx adsorbed amount per unit time can be increased in the nitrogenoxide adsorbent.

In the exhaust gas purifier according to the fifteenth aspect of theinvention, a sixteenth aspect according to the invention ischaracterized in that the nitrogen oxide adsorbent is made of transitionmetal oxide containing manganese oxide and zirconium oxide.

According to the above-mentioned configuration, because the manganeseoxide has a strong oxidizing power, the NO₂ is easily generated, and thesaturated NOx adsorbed amount and the NOx adsorbed amount per unit timecan be increased in the nitrogen oxide adsorbent.

In the exhaust gas purifier according to the sixteenth aspect of theinvention, a seventeenth aspect according to the invention ischaracterized in that a compounding ratio of manganese oxide andzirconium oxide in the nitrogen oxide adsorbent is 1:1 in terms of metalratio.

According to the above-mentioned configuration, in the transition metaloxide containing the manganese oxide and zirconium oxide, the saturatedNOx adsorbed amount of the NOx adsorbent can be increased as much aspossible.

In the exhaust gas purifier according to the sixteenth or seventeenthaspect of the invention, an eighteenth aspect according to the inventionis characterized in that the nitrogen oxide adsorbent further containsyttrium oxide.

When the yttrium oxide is added as with the above-mentionedconfiguration, because the nitrate is easily formed, the yttrium oxidecan react with the generated NO₂ to absorb the nitrogen oxide in thestate of the nitrate. Therefore, the saturated NOx adsorbed amount canfurther be increased.

In the exhaust gas purifier according to the eighteenth aspect of theinvention, a nineteenth aspect according to the invention ischaracterized in that the yttrium oxide ranges from 0.1 to 0.5 weight %in a whole of the nitrogen oxide adsorbent.

When yttrium is added in the above range, the saturated NOx adsorbedamount is extremely increased, and particularly the largest increase isobtained near 0.2 weight % in the saturated NOx adsorbed amount.

In the exhaust gas purifier according to the twentieth aspect of theinvention, a sixteenth aspect according to the invention ischaracterized in that the nitrogen oxide adsorbent further containsaluminum oxide.

The aluminum oxide is used as the support for the manganese oxide andzirconium oxide. Because the aluminum oxide is porous and has the highspecific surface area, a utilization ratio of an active site isimproved, and the saturated NOx adsorbed amount and the NOx adsorbedamount per unit time are increased.

In the exhaust gas purifier according to the twentieth aspect of theinvention, a twenty-first aspect according to the invention ischaracterized in that proportions of manganese oxide and zirconium oxidein the nitrogen oxide adsorbent range from 3 to 10 weight % in a wholeof the nitrogen oxide adsorbent.

As with the above-mentioned configuration, when the proportions of themanganese oxide and zirconium oxide range from 3 to 10 weight % in thewhole of nitrogen oxide adsorbent, the saturated NOx adsorbed amount andthe NOx adsorbed amount per unit time are increased in the nitrogenoxide adsorbent. Particularly the largest increase is obtained near 5weight %.

In the exhaust gas purifier according to the fifteenth aspect of theinvention, a twenty-second aspect according to the invention ischaracterized in that the nitrogen oxide adsorbent is made of transitionmetal oxide containing cobalt oxide and zirconium oxide.

The cobalt oxide has the strong oxidizing power similar to that of themanganese oxide described in fifteenth aspect. Thus, the NO₂ can easilybe generated to increase the NOx occluded amount by containing thecobalt oxide.

In the exhaust gas purifier according to the twenty-second aspect of theinvention, a twenty-third aspect according to the invention ischaracterized in that cobalt oxide ranges from 0.1 to 1 weight % in awhole of the adsorbent in the nitrogen oxide adsorbent.

As with the above-mentioned configuration, in the transition metal oxidecontaining the cobalt oxide and zirconium oxide, when the cobalt oxideis contained in the range of 0.1 to 1 weight % in the whole ofadsorbent, the saturated NOx adsorbed amount and the NOx adsorbed amountper unit time are increased in the nitrogen oxide adsorbent.Particularly the largest increase is obtained near 0.5 weight %.

In the exhaust gas purifier according to the fourteenth to twenty-thirdaspects of the invention, a twenty-fourth aspect according to theinvention is characterized in that a sulfur oxide adsorbent is disposedon an exhaust gas upstream side of the nitrogen oxide adsorbent.

According to the above-mentioned, because the sulfur oxide in theexhaust gas is adsorbed to the sulfur oxide adsorbent on an upstreamside of the exhaust gas before reaching the nitrogen oxide adsorbent,the nitrogen oxide adsorbent is not poisoned by the sulfur, and thedecrease in nitrogen oxide adsorbed amount caused by the sulfurpoisoning can be prevented. In addition, the durability is alsoimproved.

In the exhaust gas purifier according to the twenty-fourth aspect of theinvention, a twenty-fifth aspect according to the invention ischaracterized in that the sulfur oxide adsorbent contains copper oxideand zirconium oxide.

According to the above-mentioned configuration, the copper oxide andzirconium oxide has the good sulfur oxide (SOx) absorbing power, and canreversibly adsorb and desorb the sulfur oxide. Therefore, in the exhaustgas purifier in which the nitrogen oxide desorbing means provided on theupstream side of the nitrogen oxide adsorbent while the combustionapparatus is provided on the downstream side, by switching between thenormal operation and the regenerating operation, the nitrogen oxide isadsorbed to the nitrogen oxide adsorbent and the sulfur oxide issimultaneously adsorbed to the sulfur oxide adsorbent during theexcessive air combustion of the normal operation, and then the nitrogenoxide and sulfur oxide adsorbed to the adsorbents are desorbed duringthe over-rich combustion of the regenerating operation, and the nitrogenoxide can be detoxified on the downstream side of the combustionapparatus and discharged to the atmosphere.

In the exhaust gas purifier according to the twenty-fourth aspect of theinvention, a twenty-sixth aspect according to the invention ischaracterized in that a metal ratio a metal ratio of copper andzirconium is 1:1 in the sulfur oxide adsorbent.

According to the above-mentioned configuration, the sulfur oxideadsorbed amount can be increased.

In the exhaust gas purifier according to the twenty-fourth aspect of theinvention, a twenty-seventh aspect according to the invention ischaracterized in that the sulfur oxide adsorbent contains noble metaland lithium titanium composite oxide.

According to the above-mentioned configuration, the sulfur oxide and thenitrogen oxide can efficiently be detoxified and discharged to theatmosphere similar to the sulfur oxide adsorbent according to thetwelfth aspect in which the copper oxide and zirconium oxide are used.

Effect of the Invention

Thus, according to the invention, the lithium composite oxide containingthe lithium (Li) utilizes as the positive electrode material forlithium-ion battery and the transition metal such as the manganese (Mn)is used as the nitrogen oxide adsorbent, so that the nitrogen oxideadsorbent having the good NOx absorbing power can be provided at lowcost. Therefore, exhaust gas purifier having the large saturated NOxadsorbed amount can be provided at low cost. Particularly, theplatinum-added lithium titanate is used as the lithium titaniumcomposite oxide, or the sulfur oxide adsorbent is disposed on theexhaust gas upstream side of the nitrogen oxide adsorbent. Therefore,the poisoning of the nitrogen oxide adsorbent can be prevented.

The provision of the low-cost nitrogen oxide adsorbent made of the metaloxide containing no noble metal, particularly made of the transitionmetal oxide can decrease the apparatus cost while easily and efficientlydetoxifying and discharging the nitrogen oxide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing a first embodiment of an exhaust gaspurifier to which the invention is applied.

FIG. 2 is a schematic view showing a second embodiment of an exhaust gaspurifier to which the invention is applied.

FIG. 3 is a graph in which saturated NOx adsorbed amounts of LiMn₂O₃,LiMnPO₄, and Pt—Li₂TiO₃ are compared.

FIG. 4 is a graph showing a difference in a saturated NOx adsorbedamount between an NOx adsorbent made of Pt—Li₂TiO₃ of the invention andan NOx adsorbent made of Pt—BaO₂ system of a comparative example, and

FIG. 4 shows the case in which an SOx is contained in the exhaust gasand the case in which the SOx is not contained in the exhaust gas.

FIG. 5 is a graph showing a change in a saturated NOx adsorbed amount inthe case where a weight ratio of the NOx adsorbent made of Pt—Li₂TiO₃and a support is changed.

FIG. 6 is a graph showing a relationship between a saturated NOxadsorbed amount and an additive amount as lithium oxide (Li₂O) in theNOx adsorbent.

FIG. 7 is a graph showing a relationship between a specific surface areaand a burning temperature in producing the NOx adsorbent.

FIG. 8 is a graph showing a relationship between a saturated NOxadsorbed amount and a temperature of adsorbent desorbing means in thecase where the NOx adsorbent is burned at 450° C.

FIG. 9 is a graph showing a component ratio of substance desorbed fromthe NOx adsorbent of the present invention.

FIG. 10 is a graph showing a saturated NOx adsorbed amount of NOxadsorbents made of various transition metal oxides.

FIG. 11 is a graph showing a saturated NOx adsorbed amount in variousmetal ratios in an NOx adsorbent made of manganese oxide and zirconiumoxide.

FIG. 12 is a graph showing a change in a saturated NOx adsorbed amountto a change in additive amount of yttrium oxide in a structure theyttrium oxide is added to the NOx adsorbent made of manganese oxide andzirconium oxide.

FIG. 13 is a graph showing the change in a saturated NOx adsorbed amountto a change in ratio of the whole of a manganese oxide and zirconiumoxide in the NOx adsorbent to which aluminum oxide is added to the NOxadsorbent made of the manganese oxide and zirconium oxide and having ametal ratio of 1:1.

FIG. 14 is a graph showing the change in a saturated NOx adsorbed amountto a change in additive amount of cobalt in the NOx adsorbent made ofcobalt oxide and zirconium oxide.

FIG. 15 is a graph showing a ratio of desorbed substances of aconventional nitrogen oxide occluding catalyst containing a noble metal.

EXPLANATIONS OF LETTERS OR NUMERALS

-   -   1 internal combustion engine    -   2 exhaust passage    -   2 a and 2 b branch exhaust passage    -   2 c downstream-side exhaust passage    -   3 adsorbed substance desorbing means    -   4 NOx adsorbent    -   5 combustion apparatus    -   40 fine particle filter    -   42 SOx adsorbent    -   X1 over-rich combustion region    -   X2 lean fuel combustion region

BEST MODES FOR CARRYING OUT THE INVENTION First Embodiment of theInvention

FIG. 1 shows a first embodiment of an exhaust gas purifier according tothe invention. An exhaust passage 2 of an internal combustion engine 1or a combustion instrument is branched into first and second branchexhaust passages 2 a and 2 b, a switching valve 20 is provided in abranch portion on an upstream side of the exhaust gas, and the branchexhaust passages 2 a and 2 b are merged at an end portion on adownstream side of the exhaust gas and connected to a downstream-sideexhaust passage 2 c. The exhaust gas from the internal combustion engine1 is selectively discharged to one of the branch exhaust passages 2 aand 2 b by switching the switching valve 20, and the regeneratingoperation can be performed in the other branch exhaust passage. Examplesof the internal combustion engine 1 include a diesel engine, a gasengine, a gasoline engine, and a gas turbine engine, and an industrialboiler can be cited as an example of the combustion instrument. Theinternal combustion engine 1 and the combustion instrument are mainlyoperated under the excessive air condition.

In each of the branch exhaust passages 2 a and 2 b, an adsorbedsubstance desorbing means 3, a fine particle filter 40, a nitrogen oxideadsorbent (hereinafter referred to as “NOx adsorbent”) 4, and acombustion apparatus 5 are disposed at intervals in an exhaust gasflowing direction in the order from the exhaust gas upstream side.

The NOx adsorbent 4 is made of the lithium composite oxide expressed bya transition metal, particularly by the general formula LiAxOy orLiAxPO₄ formed by at least one kind of element A and the lithium (Li).The element A is selected from the element group of manganese (Mn),nickel (Ni), cobalt (Co), vanadium (V), chromium (Cr), iron (Fe),titanium (Ti), scandium (Sc), and yttrium (Y). Specific suitableexamples of the general formula LiAxOy include lithium manganate(LiMn₂O₄) and lithium titanate (Li₂TiO₃), and a specific suitableexample of the general formula LiAxPO₄ includes lithium manganatephosphate (LiMnPO₄).

In the present embodiment, platinum (Pt) which is of noble metal isfurther added to the lithium composite oxide, and lithium titanate(Li₂TiO₃) containing titanium (Ti) is used as the lithium compositeoxide in order to maintain the NOx absorbing power and SOx-resistantproperty at higher levels.

In the nitrogen oxide adsorbent 4, an additive amount which is of thelithium oxide Li₂O ranges from 10 to 20 weight %. The nitrogen oxideadsorbent 4 is burned in the range of 400° C. to 500° C., and preferablythe nitrogen oxide adsorbent 4 is burned at about 450° C.

In the first embodiment, the platinum and lithium titanate (Li₂TiO₃) aresupported by a support made of aluminum oxide (Al₂O₃) and/or anatasetitanium oxide (TiO₂).

The adsorbed substance desorbing means 3 disposed on the most upstreamside of the exhaust gas includes a fuel nozzle 31, an igniter 32, andair supply means 33. The fuel nozzle 31 is connected to a fuel tank 11through a fuel amount regulating apparatus 10. In the fuel nozzle 31, asupply amount and supply timing of the fuel are controlled by anelectronic control unit (hereinafter referred to as “ECU”) 12. The airsupply means 33 is connected to an air supply source 17 through an airamount regulating apparatus 16. In the air amount regulating apparatus16, the supply amount and supply timing of the fuel are controlled bythe ECU 12. For example, the over-rich combustion is performed bycontrolling the supply amount and supply timing of the fuel and thesupply amount and supply timing of the air, and the corresponding branchexhaust passage 2 a or 2 b can be heated while converted into thereducing atmosphere.

The supply amount of the fuel and supply amount of the air are set suchthat the adsorbed substance desorbing means 3 becomes the combustiontemperature near the burning temperature (400° C. to 500° C.) or theburning temperature or less. For example, when the burning temperatureis 450° C., the combustion temperature is set to a temperature near 450°C. or a temperature at 450° C. or less.

The combustion apparatus 5 disposed on the most downstream side of theexhaust gas includes a fuel nozzle 6, an igniter 7 and air supply means15. In the state in which the combustion apparatus 5 is operated, anover-rich combustion region X1 and a lean fuel combustion region X2 canbe formed on the exhaust gas upstream side and downstream side of theair supply means 15. The fuel nozzle 6 is connected to a fuel tank 11through the fuel amount regulating apparatus 10. In the fuel nozzle 6,the supply amount and supply timing of the fuel are controlled by theelectronic control unit 12. The air supply means 15 is connected to theair supply source 17 through the air amount regulating apparatus 16. Inthe air amount regulating apparatus 16, the supply amount and supplytiming of the air are controlled by the ECU 12.

In the case where the NOx adsorbent 4 does not contain the noble metalsuch as platinum, the combustion apparatus 5 controls the supply amountof the fuel and supply amount of the air such that the over-richcombustion region X1 and lean fuel combustion region X2 are formed onthe exhaust gas upstream side and downstream side of the air supplymeans 15. On the other hand, in the case where the NOx adsorbent 4contains the noble metal such as platinum like the present embodiment,the fuel supply amount is decreased and the air supply amount isincreased, and the control can be performed such that only the lean fuelcombustion region X2 is formed.

Working of the Embodiment

In the case where the internal combustion engine 1 is operated, theswitching valve 20 switches the connection of the exhaust passage 2 toutilize one of the branch exhaust passages 2 a and 2 b as the exhaustgas discharge passage of the internal combustion engine 1. Theregenerating operation is performed in the other of the branch exhaustpassages 2 a and 2 b if needed. In the state of FIG. 1, the secondbranch exhaust passage 2 b is used as the exhaust gas passage, while thefirst branch exhaust passage 2 a is used for the regenerating operation.

During the operation of the internal combustion engine 1, the combustionapparatus 5 and the adsorbed substance desorbing means 3 are stopped inthe second branch exhaust passage 2 b utilized as the exhaust gasdischarge passage in FIG. 1. Because the internal combustion engine 1 isoperated under the excessive air condition, a small amount of the CO orthe like is contained in the exhaust gas while a large amount of the NOxis probably contained in the exhaust gas. The exhaust gas flows from theexhaust passage 2 into the second branch exhaust passage 2 b, aparticulate substance is removed by a fine particle filter 40, the NOxis adsorbed to the NOx adsorbent 4, and the detoxified exhaust gas isdischarged through the exhaust passage 2 c on the downstream side.

On the other hand, the combustion apparatus 5 and the adsorbed substancedesorbing means 3 are operated in the first branch exhaust passage 2 ain which the regenerating operation is performed. In the adsorbedsubstance desorbing means 3, the fuel from the fuel nozzle 31 is burnedwith the air from the air supply means 33, whereby the high-temperatureair is supplied to the NOx adsorbent 4 to desorb the NOx from the NOxadsorbent 4. That is, the NOx adsorbent 4 is regenerated. In this case,because the combustion temperature of the adsorbed substance desorbingmeans 3 is closes to the burning temperature of the NOx adsorbent 4 ornot more than the burning temperature, the sintering and the destructionof lithium (Li) by fire are not generated.

In the case where the platinum (Pt) is added to the NOx adsorbent 4,during the regenerating operation, the NOx is reduced into the N₂ by thecatalytic action of platinum, the NOx is detoxified and discharged as inthe present invention.

In the case where the noble metal is not added to the NOx adsorbent 4,as described above, the over-rich combustion region X1 and the lean fuelcombustion region X2 are formed in the combustion apparatus 5, whichallows the NOx desorbed from the NOx adsorbent 4 to be reduced into theN₂ in the over-rich combustion region X1.

In the lean fuel combustion region X2 of the combustion apparatus 5, theCO and the hydrocarbon are oxidized and detoxified into the CO₂ and theH₂O, and are discharged. The N₂ is not oxidized because of the lowcombustion temperature in the lean fuel combustion region X2. In thecase where the noble metal is not added to the NOx adsorbent 4,sometimes the CO and the hydrocarbon are generated in the over-richcombustion region X1, and the CO and the hydrocarbon are oxidized anddetoxified in the lean fuel combustion region X2 on the downstream side.

When the NOx adsorbed amount of the NOx adsorbent 4 in the second branchexhaust passage 2 b reaches a predetermined amount (saturated amount orregulated amount less than saturated amount) by the several normaloperations, the switching valve 20 is switched to the first branchexhaust passage 2 a to stop the combustion apparatus 5 and adsorbedsubstance desorbing means 3 in the first branch exhaust passage 2 a,while the combustion apparatus 5 and adsorbed substance desorbing means3 in the second branch exhaust passage 2 b are operated. That is, thenormal operation is performed in the first branch exhaust passage 2 a,and the regenerating operation is simultaneously performed in the secondbranch exhaust passage 2 b.

Effect of the Embodiment

(1) As shown in FIG. 1, the exhaust passage 2 of the exhaust gaspurifier is branched into the two branch exhaust passages 2 a and 2 b,one of the branch exhaust passages 2 a and 2 b is used as the exhaustgas discharge passage during the normal operation, and the nitrogenoxide desorbing means 3 and combustion apparatus 5 are operated toperform the regenerating operation by blocking the other from theexhaust passage 2 of the engine. Therefore, the amount of air used forthe adsorbed substance desorption and the combustion apparatus is setirrespective of the exhaust gas amount from the internal combustionengine 1, so that the amount of fuel supplied from the adsorbedsubstance desorbing means 3 and the amount of fuel supplied in thecombustion apparatus 5 can be saved. Obviously the regeneratingoperation can be performed without performing the complicated lean andrich control on the engine side.

(2) Because the fine particle filter 40 is disposed on the upstream sideof the NOx adsorbent 4, the exhaust gas in which the particulatesubstance is removed by the fine particle filter 40 can be caused toflow in the NOx adsorbent 4 during the normal operation (used as theexhaust gas discharge passage). Accordingly, the decrease in adsorptionof the NOx adsorbent can be prevented.

(3) When the platinum (Pt) which is of the noble metal is added to theNOx adsorbent 4, the saturated NOx adsorbed amount can significantly beincreased. FIG. 3 is a graph in which the saturated NOx adsorbed amountsof the lithium composite oxides are compared among lithium manganate(LiMnO₃) to which the noble metal is not added, LiMnPO₄ to which thenoble metal is not added, and Pt—Li₂TiO₃ to which the platinum (Pt) ofthe noble metal is added. The saturated NOx adsorbed amount cansufficiently be ensured even in the lithium manganate (LiMnO₃) andlithium manganate phosphate (LiMnPO₄) to which the noble metal is notadded. However, in the case of the lithium titanate (Pt—Li₂TiO₃) towhich the platinum is added, because the saturated NOx adsorbed amountcan be ensured about twice to three times the lithium manganate (LiMnO₃)and lithium manganate phosphate (LiMnPO₄), it is found that theplatinum-added lithium titanate (Pt—Li₂TiO₃) has the good NOx adsorbingperformance.

(4) When the lithium titanate (Li₂TiO₃) is used as the NOx adsorbent 4,the NOx adsorbent 4 having a good SOx-resistant property can beprovided. Therefore, the poisoning of the NOx adsorbent 4 can beprevented to lengthen the lifetime. FIG. 4 is a graph showing theSOx-resistant property of the NOx adsorbent 4. The left two graphsaccording to the present invention indicate the NOx adsorbent 4 made ofthe platinum-added lithium titanate (Pt—Li₂TiO₃), and the right twographs which are of comparative examples indicate the NOx adsorbent 4made of platinum-added barium oxide (Pt—BaO system). Each slant-linegraph indicates the saturated NOx adsorbed amount in the case where theSOx is not contained in the exhaust gas flowing through the NOxadsorbent 4, and each cross-line graph indicates the saturated NOxadsorbed amount in the case where the SOx having 300-ppm is contained inthe exhaust gas flowing through the NOx adsorbent 4. As can be seen fromthe graph of FIG. 4, in the NOx adsorbent 4 made of platinum-addedlithium titanate (Pt—Li₂TiO₃), the saturated NOx adsorbed amount similarto that of the case in which the SOx is not contained in the exhaust gasis ensured even in the case where the SOx is contained in the exhaustgas. On the other hand, in the NOx adsorbent 4 made of platinum-addedbarium oxide (Pt—BaO system), the saturated NOx adsorbed amount islargely decreased in the case where the SOx is contained in the exhaustgas.

(5) FIG. 5 is a graph showing a change in saturated NOx adsorbed amountin the case of a change in weight ratio of the NOx adsorbent 4 made ofthe platinum-added lithium titanate (Pt—Li₂TiO₃) and the aluminum oxide(Al₂O₃) and/or titanium oxide (TiO₂) which is of a support forsupporting the platinum-added lithium titanate. When the weight ratio ofthe support ranges from 0 to 80%, the saturated NOx adsorbed amount isincreased in proportion to the increase in support. Accordingly, it ispreferably to set the weight ratio of the aluminum oxide (Al₂O₃) in therange of about 80% to approximately 95%.

(6) FIG. 6 is a graph showing a relationship between the saturated NOxadsorbed amount and an additive amount as lithium oxide (Li₂O) for theNOx adsorbent 4. As can be seen from FIG. 6, the saturated NOx adsorbedamount is substantially maintained at the maximum value when theadditive amount which is of the lithium oxide (Li₂O) ranges from 10 to20 weight %. Accordingly, the lithium oxide (Li₂O) is added in the rangeof 10 to 20 weight % in the whole of nitrogen oxide adsorbent, whichallows the saturated NOx adsorbed amount to be increased to improve theNOx absorbing power.

(7) FIG. 7 is a graph showing a relationship between a specific surfacearea, the saturated NOx adsorbed amount, and a temperature at which theNOx adsorbent 4 is burned. In FIG. 7, the burning temperatures at 450°C. and 600° C. are compared to each other. As can be seen from FIG. 7,compared with the burning temperature of 600° C., the large specificsurface area and the large saturated NOx adsorbed amount are obtained atthe burning temperature of 450° C. Therefore, the NOx adsorbent 4 burnedat 450° C. has the good NOx absorbing power.

(8) FIG. 8 is a graph showing a relationship between the saturated NOxadsorbed amount and a temperature of the adsorbed substance desorbingmeans 3 in the case where the NOx adsorbent 4 is burned at 450° C. Ascan be seen from FIG. 8, the maximum saturated NOx adsorbed amount canbe obtained near the burning temperature of 450° C., and the saturatedNOx adsorbed amount is rapidly decreased when the burning temperature ishigher than 450° C. or when the burning temperature is lower than 450°C. However, when the temperature of the adsorbed substance desorbingmeans 3 is higher than the burning temperature, because a possibility ofgenerating the sintering or destruction of lithium (Li) by fire isenhanced, it is found that the adsorbed substance desorbing means 3 isoptimally set to the temperature near the burning temperature of 450° C.during the regenerating operation.

Second Embodiment of the Invention

FIG. 2 shows a second embodiment of an exhaust gas purifier according tothe invention. When compared with the first embodiment of FIG. 1, asulfur oxide adsorbent (hereinafter referred to as “SOx adsorbent”) 42is disposed between the fine particle filter 40 and the NOx adsorbent 4.Other configurations of the second embodiment are similar to those ofFIG. 1, and identical component is designated by identical numeral.

In the second embodiment, the SOx adsorbent 42 is made of copper oxideand zirconium oxide, and a metal ratio of copper and zirconium is 1:1.

Working of Second Embodiment

The working of the second embodiment is basically similar to that of thefirst embodiment of FIG. 1 except for the working of the SOx adsorbent42, so that only the working of the SOx adsorbent 42 will be described.

During the normal operation of the internal combustion engine 1, the SOxin the exhaust gas is adsorbed to the SOx adsorbent 42. Therefore, theSOx does not flow into the NOx adsorbent 4, and the NOx adsorbent 4 canbe prevented from being poisoned by the sulfur. Particularly, in thecase where the lithium oxide except for the noble metal-added lithiumtitanate (Pt—Li₂TiO₃) is used as the NOx adsorbent 4, the SOx adsorbent42 is disposed on the upstream side of the NOx adsorbent 4 because of alow SOx-resistant property, which allows the poisoning of the NOxadsorbent 4 to be prevented.

During the regenerating operation, the high-temperature air is alsosupplied to the SOx adsorbent 42 from the adsorbed substance desorbingmeans 3 to desorb the SOx adsorbed to the SOx adsorbent 42. That is, theSOx adsorbent 42 is regenerated. The desorbed SOx is directlydischarged. As described above, because the desorbing action is alsoperformed in the NOx adsorbent 4 during the regenerating operation,there is no fear that the SOx desorbed from the SOx adsorbent 42 isre-adsorbed to the NOx adsorbent 4.

As described above, when the metal ratio of the copper and zirconium is1:1 in the sulfur oxide adsorbent 42 made of the copper oxide andzirconium oxide, the sulfur oxide adsorbed amount can be increased.

The copper oxide and zirconium oxide can reversibly adsorb and desorbthe sulfur oxide. Therefore, in the exhaust gas purifier in which thenitrogen oxide desorbing means provided on the upstream side of thenitrogen oxide adsorbent while the combustion apparatus is provided onthe downstream side, by switching between the normal operation and theregenerating operation, the nitrogen oxide is adsorbed to the nitrogenoxide adsorbent and the sulfur oxide is simultaneously adsorbed to thesulfur oxide adsorbent during the excessive air combustion of the normaloperation, and then the nitrogen oxide and sulfur oxide adsorbed to theadsorbents are desorbed during the over-rich combustion of theregenerating operation, and the nitrogen oxide can be detoxified on thedownstream side of the combustion apparatus and discharged to theatmosphere.

Third Embodiment of the Invention

Metal oxide which does not contain the noble metal is used as the NOxadsorbent 4 of the exhaust gas purifier shown in FIG. 2. Particularly,the metal oxide can efficiently adsorb the NOx in the excessive airatmosphere, and the metal oxide desorbs the adsorbed NOx when the metaloxide is heated to a predetermined temperature or when the exhaust gasis converted into the reducing atmosphere. For example, the NOxadsorbent 4 is made of the transition metal oxide containing themanganese oxide and zirconium oxide, and the compounding ratio of themanganese oxide and zirconium oxide becomes 1:1 in terms of metal ratio.

In the third embodiment, the SOx adsorbent 42 is made of the copperoxide and zirconium oxide, and the metal ratio of the copper andzirconium becomes 1:1.

Working of the Embodiment

In FIG. 2, in the case where the internal combustion engine 1 isoperated, the switching valve 20 switches the connection of the exhaustpassage 2 to utilize one of the branch exhaust passages 2 a and 2 b asthe exhaust gas discharge passage of the internal combustion engine 1.The regenerating operation is performed in the other of the branchexhaust passages 2 a and 2 b if needed. In the state of FIG. 2, thesecond branch exhaust passage 2 b is used as the exhaust gas passage ofthe internal combustion engine 1, while the first branch exhaust passage2 a is used for the regenerating operation.

During the operation of the internal combustion engine 1, the combustionapparatus 5 and the adsorbed substance desorbing means 3 are stopped inthe second branch exhaust passage 2 b utilized as the exhaust gasdischarge passage in FIG. 2. Because the internal combustion engine 1 isoperated under the excessive air condition, the small amount of the COor the like is contained in the exhaust gas while the large amount ofthe NOx is probably contained in the exhaust gas. The exhaust gas flowsfrom the exhaust passage 2 into the second branch exhaust passage 2 b,the particulate substance is removed by the fine particle filter 40, theSOx is adsorbed to the SOx adsorbent 42, the NOx is adsorbed to the NOxadsorbent 4, and the detoxified exhaust gas is discharged through theexhaust passage 2 c on the downstream side.

On the other hand, the combustion apparatus 5 and the adsorbed substancedesorbing means 3 are operated in the first branch exhaust passage 2 ain which the regenerating operation is performed. In the adsorbedsubstance desorbing means 3, the fuel from the fuel nozzle 31 is burnedwith the air from the air supply means 33, whereby the high-temperatureair is supplied to the NOx adsorbent 4 to desorb the NOx from the NOxadsorbent 4. That is, the NOx adsorbent 4 is regenerated.

The exhaust gas is blocked from the internal combustion engine 1 in thefirst branch exhaust passage 2 a in the regenerating operation state,and the first branch exhaust passage 2 a is operated independently ofthe second branch exhaust passage 2 b in the normal operation state, andthe regenerating operation is performed in the first branch exhaustpassage 2 a by the fuel supply and air supply of the adsorbed substancedesorbing means 3 and the fuel supply and air supply of the combustionapparatus 5. Therefore, the amount of air used for the adsorbedsubstance desorption and the combustion apparatus is set irrespective ofthe exhaust gas amount from the internal combustion engine 1, so thatthe amount of fuel supplied from the adsorbed substance desorbing means3 and the amount of fuel supplied in the combustion apparatus 5 can besaved. During the regenerating operation, the adsorbed substancedesorbing means 3 is burned in the over-rich combustion state, so thatthe NOx adsorbent 4 can be heated while the exhaust gas is convertedinto the reducing atmosphere. Therefore, the desorbing performance canbe improved and the NOx is not generated by the burning of the adsorbedsubstance desorbing means 3.

FIG. 9 shows a component ratio of substances desorbed from the NOxadsorbent 4 during the regenerating operation. About 8% of the whole ofthe substances is desorbed in the form of the nitrogen (N₂), and 90% ormore is desorbed in the form of the nitrogen oxide (NOx) such as the NO,the NO₂, and the N₂O. Thus, most substances are desorbed in the form ofthe NOx, the desorbed NOx is reduced into the detoxified the N₂ in theover-rich combustion region X1 of the combustion apparatus 5 on thedownstream side of FIG. 2, and the detoxified the N₂ is discharged tothe atmosphere. On the other hand, although the CO or the hydrocarbon isgenerated in the over-rich combustion region X1, the CO or thehydrocarbon is oxidized into the CO₂ in the lean fuel combustion regionX2 on the downstream side, and discharged to the atmosphere. The N₂generated in the over-rich combustion region X1 is not oxidized becauseof the low combustion temperature in the lean fuel combustion region X2.

The high-temperature air is also supplied to the SOx adsorbent 42 fromthe adsorbed substance desorbing means 3 to desorb the SOx adsorbed tothe SOx adsorbent 42. That is, the SOx adsorbent 42 is regenerated. Thedesorbed SOx is directly discharged. As described above, because thedesorbing action is also performed in the NOx adsorbent 4 during theregenerating operation, there is no fear that the SOx desorbed from theSOx adsorbent 42 is re-adsorbed to the NOx adsorbent 4.

When the NOx adsorbed amount of the NOx adsorbent 4 in the second branchexhaust passage 2 b reaches a predetermined amount (saturated amount orregulated amount less than saturated amount) by the several normaloperations, the switching valve 20 is switched to the first branchexhaust passage 2 a to stop the combustion apparatus 5 and adsorbedsubstance desorbing means 3 in the first branch exhaust passage 2 a,while the combustion apparatus 5 and adsorbed substance desorbing means3 in the second branch exhaust passage 2 b are operated. That is, thenormal operation is performed in the first branch exhaust passage 2 a,and the regenerating operation is simultaneously performed in the secondbranch exhaust passage 2 b.

Effect of the Embodiment

(1) FIG. 10 is a graph in which the saturated NOx adsorbed amount oftypical transition metal oxides are compared. The manganese Mn and thecobalt Co have the highest saturated NOx adsorbed amount, and the ironFe, the copper Cu, the nickel Ni, and the chromium Cr and the like areranked. This is because the manganese oxide and the cobalt oxide havethe strongest oxidizing power to easily generate NOx in the transitionmetal oxide. The saturated NOx adsorbed amount is substantiallyproportional to the NOx adsorbed amount per unit time. Thus, when themanganese oxide is used as the NOx adsorbent 4 like the presentembodiment, the saturated NOx adsorbed amount and the NOx adsorbedamount per unit time can be increased. Accordingly, NOx can effectivelybe adsorbed and a frequency of the regenerating operation caneconomically be decreased.

(2) FIG. 11 shows the saturated NOx adsorbed amount in various metalratios by changing the metal ratio of manganese and zirconium in thecase where the transition metal oxide (not containing the noble metal)made of the manganese oxide and zirconium oxide is used as the NOxadsorbent 4. As can be seen from the graph of FIG. 11, the highestsaturated NOx adsorbed amount (Q0) is obtained when the metal ratio ofthe manganese and zirconium (Mn:Zr) is 1:1, and then the saturated NOxadsorbed amount is decreased in the order of the metal ratio of 1:5, themetal ratio of 1:9, the metal ratio of 5:1, and the metal ratio of 9:1.Accordingly, in the present embodiment, when the metal ratio of themanganese and zirconium is set to 1:1, the large-capacity saturated NOxadsorbed amount can be ensured as the NOx adsorbent 4.

Fourth Embodiment of the Invention

Oxide of yttrium Y is added to the transition metal oxide (metal ratioof 1:1) made of the manganese oxide and zirconium oxide as the NOxadsorbent 4 of the exhaust gas purifier shown in FIG. 2. The additiveamount of yttrium oxide ranges from 0.1 to 0.5 weight % in the whole ofthe NOx adsorbent 4, and preferably the additive amount of yttrium oxideis set to about 0.2 weight %.

When the yttrium oxide is added to the nitrogen oxide adsorbent 4, thenitrate is easily formed, the yttrium oxide can react with the generatedNO₂ to absorb the nitrogen oxide in the state of the nitrate. Therefore,the saturated NOx adsorbed amount can further be increased.

FIG. 12 shows a change in saturated NOx adsorbed amount of the NOxadsorbent 4 to a change in additive amount of yttrium oxide (Y₂O₃). Inthe case where the additive amount of yttrium oxide is 0 weight %, thesaturated NOx adsorbed amount Q0 corresponds to saturated NOx adsorbedamount Q0 in which the metal ratio of FIG. 4 is 1:1. When the additiveamount of yttrium oxide is increased from 0 weight % to about 0.2 weight%, the saturated NOx adsorbed amount is rapidly increased to a maximumvalue Q3 through a value Q2 at 0.1 weight %. When the additive amount ofyttrium oxide is increased from about 0.2 weight % to about 0.5 weight%, the saturated NOx adsorbed amount is gently decreased from themaximum value Q3 to a value Q4 (<Q2) through the value Q2 at the 0.1weight %. When the additive amount of yttrium oxide exceeds about 0.5weight %, the saturated NOx adsorbed amount is kept at the value Q4, sothat there is no increase and decrease in the saturated NOx adsorbedamount. Accordingly, in order to economically increase the saturated NOxadsorbed amount, preferably the additive amount of yttrium oxide rangesfrom 0.1 to 0.5 weight % like the present embodiment, and mostpreferably the additive amount of yttrium oxide is set to about 0.2weight %.

Fifth Embodiment of the Invention

Aluminum oxide is added to the transition metal oxide (metal ratio of1:1) made of the manganese oxide and zirconium oxide in the NOxadsorbent 4 disposed in the exhaust gas purifier shown in FIG. 2. Thealuminum oxide Al₂O₃ is porous and has the high specific surface area.The aluminum oxide is used as the support, and the manganese oxide andzirconium oxide are supported by the aluminum oxide. Therefore, theutilization ratio of the active site is improved, and the saturated NOxadsorbed amount and the NOx adsorbed amount per unit time are increased.

The proportions of the manganese oxide and zirconium oxide to the wholeof the NOx adsorbent 4 range from 3 weight % to 10 weight %, andpreferably the proportions is set to about 5 weight %. In other words,the proportion of the aluminum oxide ranges from 97 to 90% in the wholeof NOx adsorbent 4, and preferably the proportion of the aluminum oxideis set to about 95%.

FIG. 13 shows a relationship between the saturated NOx adsorbed amountand a change in ratio of the manganese oxide and zirconium oxide to thewhole NOx adsorbent 4. When the manganese oxide and zirconium oxide are0 weight %, as shown in a left end of the graph, the aluminum oxide is100% and the saturated NOx adsorbed amount becomes substantially zero.When the proportions of the manganese oxide and zirconium oxide isincreased from 0 weight % to about 5 weight %, the saturated NOxadsorbed amount is rapidly increased to a maximum value Q6 through avalue Q5 at 3 weight %. When the proportions of the manganese oxide andzirconium oxide is increased from 5 weight % to about 30 weight %, thesaturated NOx adsorbed amount is decreased from the maximum value Q6 toa value Q7 through the value Q5 at the 3 weight %. When the proportionsof the manganese oxide and zirconium oxide exceeds 30 weight %, thesaturated NOx adsorbed amount is substantially kept at the value Q7.Accordingly, in order to effectively increase the saturated NOx adsorbedamount, preferably the proportions of the manganese oxide and zirconiumoxide to the whole of NOx adsorbent 4 ranges from 3 weight % to 10weight %, and most preferably the additive amount of yttrium oxide isset to about 5 weight %.

Sixth Embodiment

A transition metal oxide made of oxide of cobalt Co and oxide ofzirconium Zr is used as the NOx adsorbent 4 disposed in the exhaust gaspurifier of FIG. 2. The proportion of the cobalt oxide to the whole ofNOx adsorbent 4 ranges from 0.1 weight % to 1 weight %, and preferablythe proportion of the cobalt oxide is set to about 0.5 weight %. Thecobalt oxide has the strong oxidizing power similar to that of themanganese oxide, and the saturated NOx adsorbed amount similar to thatof the manganese oxide can be obtained by the cobalt oxide as shown inFIG. 10.

FIG. 14 shows the change in saturated NOx adsorbed amount to the changein additive amount of the cobalt oxide. When the additive amount ofcobalt oxide is increased from 0.1 weight % to about 0.5 weight %, thesaturated NOx adsorbed amount is rapidly increased from a value Q10 to amaximum value Q11. When the additive amount of cobalt oxide is increasedfrom about 0.5 weight % to about 1 weight %, the saturated NOx adsorbedamount is decreased from the maximum value Q11 to a value Q12 (>valueQ10). When the additive amount of cobalt oxide exceeds about 1 weight %,the saturated NOx adsorbed amount is substantially kept at the valueQ12. Accordingly, in order to economically increase the saturated NOxadsorbed amount, preferably the additive amount of cobalt oxide rangesfrom 0.1 to 1 weight %, and most preferably the additive amount ofcobalt oxide is set to about 0.5 weight %.

Seventh Embodiment of the Invention

A sulfur oxide adsorbent 42 made of noble metal and lithium titaniumcomposite oxide (Li/TiO₂) is provided as the SOx adsorbent 42 disposedin the exhaust gas purifier of FIG. 2. Examples of the noble metalinclude platinum Pt and rhodium Rh or the like.

As with the sulfur oxide adsorbent made of the manganese oxide andzirconium oxide of the first embodiment, the sulfur oxide adsorbent 42made of the noble metal and lithium titanium composite oxide has thegood SOx absorbing power and is able to reversibly adsorb and desorb theSOx. Therefore, as shown in FIG. 2, the adsorption and desorption can beperformed without performing the lean and rich control on the engineside in the exhaust gas purifier including the adsorbed substancedesorbing means 3 and combustion apparatus 5.

INDUSTRIAL APPLICABILITY

The present invention can be used as the exhaust gas purifier forvarious machines which discharge the exhaust gas, and the invention canbe applied to the internal combustion engines such as the diesel engine,the gas engine, the gasoline engine, and the gas turbine engine or thecombustion instruments such as the incinerator and the boiler.

1. An exhaust gas purifier being installed in an exhaust passage of aninternal combustion engine or a combustion instrument, comprising: theexhaust gas purifier characterized in that a nitrogen oxide adsorbent isdisposed in the exhaust passage, and the nitrogen oxide adsorbent ismade of lithium composite oxide of Lithium manganate (LiMn₂O₄) orLithium manganate phosphate (LiMn₂O₄).
 2. The exhaust gas purifieraccording to claim 1, wherein noble metal is added to the nitrogen oxideadsorbent.
 3. The exhaust gas purifier according to claim 2, whereinnoble metal is platinum (Pt).
 4. The exhaust gas purifier according toclaim 1, wherein the nitrogen oxide adsorbent is supported by a supportmade of at least one of aluminum oxide (Al₂O₃) and anatase titaniumoxide (TiO₂).
 5. The exhaust gas purifier according to claim 1, whereinan additive amount as lithium oxide (Li₂O) ranges from 10 to 20 weight %in the nitrogen oxide adsorbent.
 6. The exhaust gas purifier accordingto claim 1, wherein nitrogen oxide adsorbent is burned in a range of400° C. to 500° C.
 7. The exhaust gas purifier according to claim 2,wherein adsorbed substance desorbing means is disposed on an exhaust gasupstream side of the nitrogen oxide adsorbent and a combustion apparatusis disposed on an exhaust gas downstream side of the nitrogen oxideadsorbent.
 8. The exhaust gas purifier according to claim 7, wherein thecombustion apparatus is a fuel lean-burn type.
 9. The exhaust gaspurifier according to claim 1, wherein the adsorbed substance desorbingmeans is set to a temperature near the burning temperature or to theburning temperature or less.
 10. The exhaust gas purifier according toclaim 7, wherein the adsorbed substance desorbing means is set to atemperature near the burning temperature or to the burning temperatureor less.
 11. The exhaust gas purifier according to claim 1, wherein asulfur oxide adsorbent is disposed on an exhaust gas upstream side ofthe nitrogen oxide adsorbent.
 12. The exhaust gas purifier according toclaim 11, wherein the sulfur oxide adsorbent contains copper oxide andzirconium oxide.
 13. The exhaust gas purifier according to claim 12,wherein a metal ratio of copper and zirconium is 1:1 in the sulfur oxideadsorbent.
 14. The exhaust gas purifier according to claim 11, whereinadsorbed substance desorbing means is disposed on an exhaust gasupstream side of the sulfur oxide adsorbent and a combustion apparatusis disposed on an exhaust gas downstream side of the nitrogen oxideadsorbent.
 15. The exhaust gas purifier according to claim 12, whereinadsorbed substance desorbing means is disposed on an exhaust gasupstream side of the sulfur oxide adsorbent and a combustion apparatusis disposed on an exhaust gas downstream side of the nitrogen oxideadsorbent.
 16. The exhaust gas purifier according to claim 13, whereinadsorbed substance desorbing means is disposed on an exhaust gasupstream side of the sulfur oxide adsorbent and a combustion apparatusis disposed on an exhaust gas downstream side of the nitrogen oxideadsorbent.